Cathode Porosity Research Articles

Numerical analysis and experimental research on cathode pore-forming characteristics and mass transfer performance of direct ethanol fuel cell

Design of direct ethanol fuel cell (DEFC) plays a key role in its viability. A novel tubular DEFC is proposed to reconstruct the conventional structure, and tubular cathode is prepared by gel injection molding with Mesocarbon microbeads (MCMB), graphite and NH4Cl with different contents as raw materials. Moreover, simulation on the mass transfer and electrochemical performance of DEFC with a wider range of cathode porosity is carried out. Experiment results indicates that the support has higher porosity and strength when the graphite doping and NH4Cl is 40% and 2%–10%, respectively. Furthermore, superior performance of cathode and DEFC electrical performance are perceived due to larger porosity, especially when the porosity is 0.5. Moreover, in the simulation the results also show that porosity of cathode diffusion layer (CCL) has a significant effect on the oxygen mass transfer and removal of liquid water. Larger porosity means better electrical performance and delivers more power, which is more obvious at 0.2–0.6. It is proved that when the porosity is about 0.5, comprehensive performance of tubular DEFC is better than other models.

Effects of cathode thickness and microstructural properties on the performance of protonic ceramic fuel cell (PCFC): A 3D modelling study

Protonic Ceramic Fuel Cells (PCFCs) are promising power sources operating at an intermediate temperature. Although plenty of experimental studies focusing on novel material development are available, the design optimization of PCFC through numerical modelling is limited. In this study, a 3D PCFC model focusing on the cathode thickness and microstructure design is developed due to the high overpotential loss of the cathode. Unlike the 1D/2D models, the rib-size effects on the PCFC performance are fully considered when optimizing the cathode structure. Different from 1D/2D models suggesting thin cathode thickness, this study finds that the optimal cathode thickness is about 120–200 μm. In a thin cathode, weak O2 diffusion from the channel to the rib-covered cathode can lead to O2 depletion under the rib and very low local cell performance. By adjusting the cathode porosity from 0.3 to 0.5, nearly 9% performance improvement and 22.5% improvement in gas distribution uniformity can be achieved. When the cathode particle size changes from 0.1 μm to 0.2 μm, the O2 concentration under the rib increases nearly 50%. The optimal electronic phase volume fraction is suggested to be around 50–60% for achieving a balance between ohmic resistance and reaction sites. This model elucidates the relationship between cathode microstructure and PCFC performance comprehensively and can serve as a guiding tool for cell fabrication and future novel interconnect structure design.

Quasi-Solid-State Lithium Metal Batteries Using the LiNi0.8Co0.1Mn0.1O2-Li1+xAlxTi2-x(PO4)3 Composite Positive Electrode.

NASICON-type Li1+xAlxTi2-x(PO4)3 (LATP) is a promising solid electrolyte (SE) candidate for next-generation solid-state batteries. However, its use in solid-state composite electrodes is inhibited by its stiffness, which results in poor interparticle contact unless high-temperature treatments are applied. The poor LATP-LATP and LATP-active material in the positive electrode (cathode) composite produced at ambient temperature yield poor ionic conductivity, impeding the electrode's performance. Herein, we focus on the optimization of the electrochemical performance of LiNi0.8Co0.1Mn0.1O2 (NCM811)-LATP composite electrodes made by tape casting, taking advantage of a small fraction of an ionic liquid electrolyte (ILE) filling the composite cathode porosity. The incorporated LATP particles are found to closely surround the large NCM811 secondary particles, partially filling the composite electrode pores and resulting in a porosity reduction from 37 vol % (NCM811 only) to 32 vol % (NCM811-LATP). After filling up the majority of the electrode porosity with ILE, the NCM811-LATP composite electrodes offer improved capacity retention upon both long-term cycling tests (>99.3% after 200 cycles) and high-rate tests (>70% at 2 C-rate), due to the more stable LATP|NCM811 interface, and facilitated Li+ diffusion in the composite electrode bulk. Results obtained from proof-of-concepts monopolar (3.0-4.3 V) and bipolar-stacked (6.0-8.6 V) cells are reported.

Microalgae hydrogel-derived monolithicfree-standing air cathode for microbial fuel cells: Tailoring the macroporous structure for enhanced bioelectricity generation

A high-performance and cost-effective cathode is highly desirable for the practical implementation of microbial fuel cell (MFC) technology, used for renewable energy recovery from wastewater. In this study, a monolithic, free-standing air cathode for MFCs is fabricated through a sol-gel-based route. Chlorella pyrenoidosa (CP) is selected as the catalyst precursor, polyacrylonitrile fiber as the precursor for the conductive and supporting skeleton, and deionized water as the solvent and pore former. Agar is also used as the gelling agent to facilitate the formation of 3D porous networks inside the cathodes. By systematically tailoring the porosity of the tubular air cathodes by varying the water content in the CP agar composite hydrogel, the mechanical strength and electrical conductivity of the cathode is optimized. Moreover, this fabrication process leads to abundant active sites for the oxygen reduction reaction and an interconnected hierarchical porous structure with separated transport paths for air supply and ion transfer. The tubular air cathode can be directly employed as an MFC cathode without the use of a polymer binder or other modifications. The MFC delivers a maximum power density of 106.3 ± 5.9 W m-3, which is approximately 33% higher than that with a converntional Pt/C cathode (79.8 ± 4.1 W m-3). This work therefore offers a promising new route for recovering bioelectricity from wastewater based on MFCs.

Ultralight Electrolyte for High‐Energy Lithium–Sulfur Pouch Cells

AbstractThe high weight fraction of the electrolyte in lithium–sulfur (Li–S) full cell is the primary reason its specific energy is much below expectations. Thus far, it is still a challenge to reduce the electrolyte volume of Li–S batteries owing to their high cathode porosity and electrolyte depletion from the Li metal anode. Herein, we propose an ultralight electrolyte (0.83 g mL−1) by introducing a weakly‐coordinating and Li‐compatible monoether, which greatly reduces the weight fraction of electrolyte within the whole cell and also enables Li–S pouch cell functionality under lean‐electrolyte conditions. Compared to Li–S batteries using conventional counterparts (≈1.2 g mL−1), the Li–S pouch cells equipped with our ultralight electrolyte could achieve an ultralow electrolyte weight/capacity ratio (E/C) of 2.2 g Ah−1 and realize a 19.2 % improvement in specific energy (from 329.9 to 393.4 Wh kg−1) under E/S=3.0 μL mg−1. Moreover, more than 20 % improvement in specific energy could be achieved using our ultralight electrolyte at various E/S ratios.

Three-Dimensional Analysis for LiNi1/3Co1/3Mn1/3O2 Composite Cathode of All-Solid-State Batteries By X-Ray Computed Tomography

All-solid-state batteries (ASSBs) using inorganic solid electrolytes are expected to achieve high safety and high energy density as alternatives to conventional lithium-ion batteries using liquid electrolytes. However, in many cases, rate capability of ASSBs is poor compared with that of the conventional liquid system. This is because the contact interface between solid active materials and solid electrolyte is insufficient, resulting in high resistance for electrochemical properties. The solid electrode-electrolyte interface in ASSBs is related to the morphological structure such as porosity and mixed state. It has been confirmed that pressurization reduces the grain boundaries of solid electrolytes and improves conductivity1). From this improvement, it is generally known that applying high pressure when assembling ASSBs improves the contact area of the solid-solid interface, including electrode and electrolyte interface. However, the effect of pressure on particle-particle contact and battery performance is not understood enough. In this study, we investigated the effect of pressure-induced three-dimensional structure on the electrochemical properties of ASSBs by using X-ray computed tomography (CT). ASSBs were prepared at various stack pressures using a composite cathode consisting of oxide active materials LiNi1/3Co1/3Mn1/3O2 (NCM), sulfide solid electrolyte Li10GeP2S12 (LGPS), and acetylene black. Electrochemical properties were investigated in ASSBs of NCM composite cathode|LGPS solid-electrolyte|In-Li anode. The charge–discharge measurements were performed using various stack pressure, which indicates that the capacity increases as the stack pressure increases. To analyze the factors behind this result in terms of the internal structure of the ASSBs, we performed X-ray CT with synchrotron radiation at 20 keV using a homemade operando measurement cell that electrode diameter is 1 mm. This measurement system allows us to analyze the three-dimensional structure of composite cathode during monitoring applied pressure, electrochemical measurements, and X-ray CT measurements. From the X-ray CT data, we succeeded in quantifying the changes in the porosity of the composite cathode and the contact state between the active material and electrolytes by changing the pressure. Quantitative analysis revealed that the porosity and contact state correlate with the charge–discharge properties. Additionally, CT analysis revealed that the voids are preferentially presented near active materials and these voids spread the horizontal direction (not pressurization direction), resulting in different porosities of composite cathode and solid electrolyte layer. Since it was confirmed that voids remained in the vicinity of the active material even when the pressurization was continued, controlling these voids decreases the voids of the entire cathode layer. Pressure dependencies on AC impedance during X-ray CT measurements showed that the apparent conductivity of ASSBs increases linearly as the confinement stack pressure increases. This result is caused by suggesting that the decrease in porosity and tortuosity in cathode layer, which is detected by CT analysis. Additionally, date of tortuosity shows over-pressurization may bypass Lithium conduction path. The improved internal structure such as contact state due to pressurization enhances the apparent conductivity and charged-discharge characteristics of ASSBs. With the latest synchrotron radiation measurement technology and image processing technology, high-precision analysis has become possible.1) Jean-Marie Doux, Yangyuchen Yang, Darren H. S. Tan, Han Nguyen, Erik A. Wu, a Xuefeng Wang, Abhik Banerjeea and Ying Shirley Meng, J. Mater. Chem. A, 8, 5049, (2020).

Microstructure characterization and thermionic emission performance of barium-tungsten cathode

The effect of the parameters (sintering temperature, pressure, holding time) of spark plasma sintering technique on the W porosity of Ba-W cathode was studied by orthogonal method. When the W porosity varied in the range of 23%−30%, the corresponding process parameters were obtained. On this basis, the spherical and traditional irregular W matrix with different porosity were sintered. The results show that the spherical porous W particles were packed and arranged orderly, and the pore size distribution was concentrated and uniform. When the porosity of spherical W was 26.3%, the size of the median pore was 1.41 μm. The vickers hardness of spherical tungsten matrix was lower than that of conventional irregular tungsten matrix. Under the condition of pulse width 10 μs and frequency 1000 Hz, the impulse current density of Ba-W cathode increased at first and then decreased with the increase in porosity. The maximum current density belonged to the Ba-W cathode with the matrix porosity of 26.3%. At 1050 ℃, the off-point emission current density of Ba-W cathode reached 24.62 A/cm2, the corresponding zero-field current density and workfunction were 7.62 A/cm2 and 1.95 eV, respectively.

Improvement of air cathode performance in microbial fuel cells by using catalysts made by binding metal-organic framework and activated carbon through ultrasonication and solution precipitation

Activated carbon (AC) is an inexpensive catalyst for the oxygen reduction reaction in the air cathode of microbial fuel cells (MFCs). However, since the electrochemical catalytic activity of AC is poor, it is necessary to improve its performance. The metal–organic framework (MOF) is composed of a metal ion and an organic linker. It has high porosity and high electrochemical catalytic activity. Herein, ZIF-67 (cobalt-nitrogen framework) was combined with activated carbon through ultrasonication (U) and solution precipitation (H), which was used to make ZIF-67U and ZIF-67H cathodes, respectively. In maximum power density, ZIF-67U cathode produced 4203 mW m−2, and ZIF-67H did 3881 mW m−2, which is 60% and 48% higher than AC cathode (2625 mW m−2) and 160% and 140% higher than Pt cathode (1614 mW m−2), respectively. Cobalt and nitrogen contents increased in the ZIF catalysts. In atomic nitrogen contents of catalyst surface, pyridine-N was 28% in ZIF-67U and 38% in ZIF-67H, respectively; pyrrole-N was 56% in ZIF-67U and 25% in ZIF-67H, respectively; no nitrogen was detected in AC. These cobalt-nitrogen increased the active site of the oxygen reduction reaction (ORR), improved the reaction rate, and decreased charge transfer impedance. Impedance analysis demonstrated the ZIF-67 addition also decreased the diffusion impedance possibly due to the improved cathode porosity. AC and ZIF-67 were bonded using ultrasonication and tested in the MFC for the first time, producing the highest power ever among the MOFs in the 50-mM phosphate-buffer-saline condition so far.

Dense-Stacking Porous Conjugated Polymer as Reactive-Type Host for High-Performance Lithium Sulfur Batteries.

Commercialization of the lithium-sulfur battery is hampered by bottlenecks like low sulfur loading, high cathode porosity, uncontrollable Li2 Sx deposition and sluggish kinetics of Li2 S activation. Herein, we developed a densely stacked redox-active hexaazatrinaphthylene (HATN) polymer with a surface area of 302 m2 g-1 and a very high bulk density of ca. 1.60 g cm-3 . Uniquely, HATN polymer has a similar redox potential window to S, which facilitates the binding of Li2 Sx and its transformation chemistry within the bulky polymer host, leading to fast Li2 S/S kinetics. The compact polymer/S electrode presents a high sulfur loading of ca. 15 mgs cm-2 (200-μm thickness) with a low cathode porosity of 41 %. It delivers a high areal capacity of ca. 14 mAh cm-2 and good cycling stability (200 cycles) at electrolyte-sulfur (E/S) ratio of 5 μL mgs -1 . The assembled pouch cell delivers a cell-level high energy density of 303 Wh kg-1 and 392 Wh L-1 .

Dense‐Stacking Porous Conjugated Polymer as Reactive‐Type Host for High‐Performance Lithium Sulfur Batteries

AbstractCommercialization of the lithium‐sulfur battery is hampered by bottlenecks like low sulfur loading, high cathode porosity, uncontrollable Li2Sx deposition and sluggish kinetics of Li2S activation. Herein, we developed a densely stacked redox‐active hexaazatrinaphthylene (HATN) polymer with a surface area of 302 m2 g−1 and a very high bulk density of ca. 1.60 g cm−3. Uniquely, HATN polymer has a similar redox potential window to S, which facilitates the binding of Li2Sx and its transformation chemistry within the bulky polymer host, leading to fast Li2S/S kinetics. The compact polymer/S electrode presents a high sulfur loading of ca. 15 mgs cm−2 (200‐μm thickness) with a low cathode porosity of 41 %. It delivers a high areal capacity of ca. 14 mAh cm−2 and good cycling stability (200 cycles) at electrolyte–sulfur (E/S) ratio of 5 μL mgs−1. The assembled pouch cell delivers a cell‐level high energy density of 303 Wh kg−1 and 392 Wh L−1.

Solid oxide fuel cell cathode diffusion polarization: materials and exergy study

The efficiency of the solid oxide fuel cell (SOFC) still can be enhanced by decreasing the cathode diffusion polarization, however to resolve this problem, band gap materials engineering analysis for the SOFC cathode must be well-thought-out. In this research, we investigated theoretically and analytically the effect of several materials constraints and functional conditions on the air electrode diffusion polarization and its influence on exergy efficiency of the fuel cell. The novel model of this paper is the second part of the model created on the SOFC to investigate the polarization on the electrodes and electrolyte. The model is divided into a number of units axially; for each unit, the thermal equations and the continuity equations of the electrochemical reactions are progressively solved with an iterative approach. The partial differential equations for mass and energy transfer through the cathode were spatially discretized using the finite differences method (FDM). The results shows that in order to maintain the peak exergy efficiency of the SOFCs in applied application environments, it is suggested to have the SOFC air cathode porosity between 0.35 and 0.38, the cathode pore mean radius in the range of 0.00015 to 0.0002 cm, the cathode material tortuosity of 2.9, the SOFC functional temperature between 850 °C and 950 °C, the thickness of the air electrode between 0.01 and 0.02 cm, and have the operating pressure in the range of 3 and 5 bar.

Optimization of the cathode porosity via mechanochemical synthesis with carbon black

Nanostructured carbon–coated composite cathode materials LiFe0.5Mn0.5PO4/C (LFMP/C) are prepared by the mechanochemically assisted solid-state synthesis using different reagent mixtures and carbon as reducing and covering agent. The effect of the precursors, gas release during the solid-state reaction, and of the intensity of high-energy ball milling on the porous structure and electrochemistry of LFMP/C is studied using DSC/TG/MS, XRD, SEM, TEM, standard contact porosimetry (MSCP), EIS, CV, and GVC. It is shown that the particle size and porosity of LFMP/C strongly depend on the chosen precursors and intensity of mechanical impact. The higher the intensity, the more effective incorporation of carbon black in the pores formed in LFMP, which leads to improved electronic conductivity and better access of the electrolyte to the surface of the electrode, while smaller particles provide improved Li diffusion in the bulk of LFMP. As a result, the cyclability and high-rate performance of the LFMP/C composites are improved.

Modeling the concentration profiles of CO and CO2 in a direct carbon solid oxide fuel cell—effect of cathode and electrolyte design parameters

A mathematical model is developed to study the concentration profile of gases, namely carbon dioxide and carbon monoxide, across the anode of a direct carbon solid oxide fuel cell (DC-SOFC) under anode-supported configuration. The concentration profiles are evaluated as a function of cathode and electrolyte design parameters, namely cathode porosity, cathode tortuosity, cathode composition, cathode pressure, electrolyte thickness, and length of the triple-phase boundary (TPB). The effect of operating potential on these concentration profiles is modeled. The effects of electrolyte thickness and TPB length are found to be major factors in determining the performance of the DC-SOFC. Model results are compared with experimental data and found to compare well.

Space-confined construction of nitrogen-rich cobalt porphyrin-derived nanoparticulates anchored on activated carbon for high-current lithium thionyl chloride battery

Primary batteries based on lithium metal anodes have been attracting increasing attention due to their high capacity and energy density, but the implementation high-current of battery still faces many challenges, such as low reaction rate of SOCl2 and large impedance. Space-confined construction of nitrogen-rich cobalt porphyrin-derived nanoparticulates anchored on functional-activated carbon (NCA) composites were prepared by in situ solid phase synthesis as catalysts of carbon cathode to address these issues. On the molecular level, the π-π conjugate system is further enhanced by forming C–O–Co bond between the Co atoms of cobalt porphyrin derivatives and the oxygen-containing functional groups of functional-activated carbon. Significantly, the resulting NCA composites exhibit the longest high-current (above 25 mA/cm2) discharge time, up to 37 min, and the capacity is about 19.18 mAh, accounting for 88.5% of its total capacity. Meanwhile, the internal resistance of the battery catalyzed by the NCA composites (24.06 Ω) is 0.40 times lower than that without catalysts, and the reduction peak of cyclic voltammetry is the largest about 2.357 V, showing that the electrode with NCA composites has good catalytic activity. All this is due to the fact that the carrier activated carbon of the NCA composites reduces the internal resistance of the battery by regulating the porosity of the carbon cathode, and the loaded CoTAP nanoparticulates improve the reduction rate of SOCl2.

Impact of Polyvinylidene Fluoride on Nanofiber Cathode Structure and Durability in Proton Exchange Membrane Fuel Cells

The impact of polyvinylidene fluoride (PVDF) as a binder component on the durability of Pt/C cathodes in a proton exchange membrane fuel cell membrane-electrode-assembly (MEA) during a carbon corrosion accelerated stress test (AST) was examined using electrochemical fuel cell data and visual inspection/analysis of the cathode morphology via electron-microscopy. Electrospun nanofiber cathode mat MEAs with a Nafion®/PVDF or Nafion/poly(acrylic acid) (PAA) binder or a slurry cathode MEA with neat Nafion or a Nafion/PVDF binder were investigated. The presence of PVDF had profound effects on the structure and chemical/electrochemical properties of a fuel cell cathode; its hydrophobic property slowed the rate of carbon loss and its robust mechanical properties added strength to the binder. Thus, the extent of carbon loss during an AST was inversely proportional to the PVDF content of the binder and there was no observable cathode thinning nor any change in cathode porosity after the AST, when the cathode binder contained at least 50 wt% PVDF. In terms of long-term durability, these beneficial structural effects outweighed the lower Nafion/PVDF binder conductivity and the associated lower initial power output of a Nafion/PVDF cathode MEA. For hydrophilic slurry and nanofiber cathodes with neat Nafion or Nafion/PAA fibers, low power after the carbon corrosion AST was due to greater carbon losses, cathode thinning and the collapse of cathode pores, which dominated MEA performance even though the initial cathode ECSA and mass activity were high for these two MEAs.

A novel interconnector design of SOFC

In this study, a novel interconnector design is proposed, which is named as the X-type interconnector. The solid oxide fuel cell (SOFC) models are established for the conventional interconnector and the X-type interconnector. The results indicate that the design of the X-type interconnector is beneficial to the transport of gas in SOFC, which has a higher oxygen concentration under rib than that of the conventional interconnector. Furthermore, compared with the X-type interconnector, the potential difference in cathode is larger for the conventional interconnector, which indicates that the X-type interconnector reduces the current path and improves the performance of SOFC. For any porosity and conductivity of anode and cathode, the X-type interconnector is superior to the conventional interconnector. Moreover, when the cathode conductivity is smaller, the advantage of the X-type interconnector becomes more remarkable.

The analysis of structure parameters of MOLB type solid oxide fuel cell

A mono-block-layer build solid oxide fuel cell model (MOLB type SOFC) is established, which takes into account the contact resistance at the electrode/interconnector interface due to the produce of the oxide scale. The influence of structure parameter on the MOLB type SOFC performance is investigated, which includes that the anode thickness, cathode thickness, the anode/interconnector interface width, the cathode/interconnector interface width, the channel height, anode porosity and cathode porosity. Although most researchers reported that the performance of MOLB type SOFC was superior to that of planar type SOFC, this work revealed that at the condition of the lower cathode thickness (<100 μm), the MOLB type SOFC performance was worse than that of the planar type SOFC. Moreover, for the MOLB type SOFC, the power density per unit area slightly rises with the increase of channel height, but the power density per unit volume sorely drops with the increase of channel height. Consequently, the structure parameters of the MOLB type SOFC affect the cell performance significantly.

Cathode porosity is a missing key parameter to optimize lithium-sulfur battery energy density

While high sulfur loading has been pursued as a key parameter to build realistic high-energy lithium-sulfur batteries, less attention has been paid to the cathode porosity, which is much higher in sulfur/carbon composite cathodes than in traditional lithium-ion battery electrodes. For high-energy lithium-sulfur batteries, a dense electrode with low porosity is desired to minimize electrolyte intake, parasitic weight, and cost. Here we report the profound impact on the discharge polarization, reversible capacity, and cell cycling life of lithium-sulfur batteries by decreasing cathode porosities from 70 to 40%. According to the developed mechanism-based analytical model, we demonstrate that sulfur utilization is limited by the solubility of lithium-polysulfides and further conversion from lithium-polysulfides to Li2S is limited by the electronically accessible surface area of the carbon matrix. Finally, we predict an optimized cathode porosity to maximize the cell level volumetric energy density without sacrificing the sulfur utilization.

Distribution of relaxation times analysis and interfacial effects of LSCF fired at different temperatures

This paper describes the effects of (La0.60Sr0.40)0.95Co0.20Fe0.80O3-x (LSCF) firing temperature on the electrochemical performance and the chemical interactions at the interface of Gd-doped ceria (GDC) barrier layer and yttria-stabilized zirconia (YSZ) electrolyte for commercial cells. Initial scanning electron microscopy (SEM) analysis indicate that the optimal cathode porosity is obtained at a firing temperature of 1100 °C; however, the peak power is not observed at the same firing temperature. At 750 °C and using humidified hydrogen, the peak power densities are found to be 0.607, 0.959, 1.41, and 0.842 W/cm2 for a cathode firing temperature of 900, 1000, 1050, and 1100 °C, respectively. The power density data indicate an increasing trend in performance as the firing temperature increases; however, the electrochemical performance drops drastically at a firing temperature of 1100 °C. Using impedance spectroscopy (IS) and distribution of relaxation times (DRT) analysis, it is determined that the decrease in cell performance is mostly due to a slower charge transfer process. This is further confirmed using high resolution SEM/EDS and STEM/EDS analysis which show that Sr is present at the GDC/YSZ interface with the highest concentration at 1100 °C. This indicates that an insulating layer of strontium zirconate (possibly SrZrO3) has formed and is mostly responsible for low cell performance. In addition, STEM/EDS analysis shows that Zr and Ce have interdiffused with each other as well. It is further shown that more Zr (in addition to interdiffusion) has moved into the barrier layer in order to react with Sr to form the strontium zirconate.

Effects of Sulfur Loading, Cathode Porosity, and Electrolyte Amount on Li-S Battery Performance with Solvate Ionic Liquid Electrolyte

Effects of Sulfur Loading, Cathode Porosity, and Electrolyte Amount on Li-S Battery Performance with Solvate Ionic Liquid Electrolyte